Undo courtesy accidental in select key signatures causes crash

1. 'Create Score'.
2. At the key signature step, choose one of the following: D major, A major or C♯ major.
3. 'Finish'.
4. Enable 'Note Entry'.
5. Click on C (sharp).
6. Click on ♯ in the toolbar.
7. 'Undo'.

OR:

1. 'Create Score'.
2. At the key signature step, choose E♭ major.
3. 'Finish'.
4. Enable 'Note Entry'.
5. Click on B (flat).
6. Click on ♭ in the toolbar.
7. 'Undo'.

Result: Crash.

Using MuseScore 2.0 Nightly Build (5663) - Mac 10.7.4.